Innate immune responses are dictated by a panel of pathogen recognition receptors, downstream signaling from the receptors and the stimulated activities of various effector molecules. IRF8 is known as an interferon (IFN)-responsive transcription factor that plays critical roles in regulating the development of myeloid and dendritic cells and the activity of a number of genes, such as IL-12 and iNOS, involved in innate responses. Much of the activity of IRF8 in vitro was previously shown to require its ability to heterodimerize with PU.1 and other transcription factors to mediate transcriptional activation or repression. An in vivo test of this model was provided by studies of BXH2 mice that identified a point mutation in IRF8 in the domain required for heterodimerization. It was shown that mice bearing this mutation were very similar to those bearing a null mutation of the gene, but that the null and point-mutant mice differed in their patterns of dendritic cell maturation. This indicated that most, but not all in vivo activities of IRF8 are dependent on its ability to dimerize with other transcription factors. Previous studies demonstrated that IRF8 is expressed to varying extents in cells of bone marrow origin. Recent evidence indicates broader than expected expression patterns of IRF8 in other non-hematopoietic tissues. To permit examinations of IRF8 expression on a single cell basis, we generated an IRF8 reporter mouse that expresses an IRF8-EGFP fusion protein under the control of normal endogenous IRF8 regulatory sequences. Expression levels were found to vary widely during various stages of hematopoietic differentiation with hematopoietic stem cells expressing little if any while dendritic cells expressed very high levels. Importantly, examining levels of IRF8-EGFP expression made it possible to define three subsets of what was previously thought to be a homogeneous population of granulocyte-myeloid progenitors. Interestingly, we found that IRF8 is expressed in gastric mucosa. These findings provide new insights into the expression and functions of IRF8 in a variety of tissues. Studies of a mouse model of multiple sclerosis showed that development of disease was completely dependent on expression of IRF8. We found that expression of IRF8 in antigen-presenting cells promoted disease onset and progression through multiple mechanisms. These included induction of a cytokine environment that furthered the development of Th2 and Th17 inflammatory cells. IRF8 also activated microglia and exacerbated neuroinflammation. These studies provide a basis for understanding the role of IRF8 in the pathogenesis of multiple sclerosis. Autophagy is the basic physiological process that deals with cell degradation caused by normal cell turnover or stress. It plays a critical role in innate immune responses. We found that IRF8 is up-regulated in dendritic cells and macrophages by many autophagy-inducing stresses. As a result, IRF8 regulates expression of many autophagy-related genes, promotes autophagosome formation and lysosomal fusion. Deficiency of IRF8 in macrophages caused defective autophagy activity and poor clearance of Listeria antigens. This study revealed a novel mechanism of IRF8 in regulation of innate immune responses. Basophils and mast cells are important effectors of the immune system in host defense against pathogens and allergic diseases. We found that IRF8 plays an essential role in development of basophil and mast cells. Similar to the role of IRF8 in early lymphoid and myeloid lineage progenitor cell differentiation, IRF8 regulates a gene program required for development of pre-basophil and mast cell progenitors by modulating expression of GATA2, a transcription factor known to facilitate basophil and mast cell differentiation. These results shed new light on the functions of IRF8 in controlling lymphoid and myeloid lineage specification and commitment. Moreover, our assessment of IRF8 in T cells demonstrated that IRF8 suppresses GM-CSF expression in T cells and affects differentiation of tumor-induced myeloid-derived suppressor cells. These findings suggest that IRF8 not only is required for development of normal myeloid cells but also plays a role in generation of pathogenic myeloid cells. We also studied the transcriptional network involved in the development of plasma cells. PU.1 is a known partner of IRF8 and both transcription factors are expressed at high levels in B cells. However, deletion of either PU.1 or IRF8 in B lineage cells has no obvious effect on B cell biology. To better understand the functions of PU.1 and IRF8 in B cells, we have generated and studied PU.1 and IRF8 double conditional deletion mice using CD19-Cre and Mb1-Cre. We found that IRF8/PU.1 controls class-switch recombination and plasma cell differentiation by promoting the expression of BCL6 and PAX5 and suppressing BLIMP-1. This study combined with our another finding that IRF8 interacts with BCOR (B cell lymphoma 6 (BCL6) corepressor) revealed a new transcriptional regulatory network involving IRF8, PU.1 and BCOR in modulating the expression of germinal center and post germinal center gene programs.